1. Field of the Invention
The present invention relates to radiation imagery chemistry in which imaging affects a physical property of a radiation sensitive material, or produces a nonplanar or printing surface, by electron beam imaging. More specifically, the invention relates to the formation of localized oxide growth on metal films in the presence of electron beams and high vacuum level pressures of gases such as CO.sub.2 or N.sub.2 O that produce stable leaving groups.
2. Description of the Prior Art
Highly resolved structures are of value in a variety of technologies, such as in the manufacture of integrated circuits, mass storage devices, and micromechanical devices. For example, an integrated circuit consisting of a network of interconnected transistors on a substrate is formed using photographic and microlithographic techniques to define features as small as 2-5 micrometers. The smallness of the features accounts for the useful physical attributes of such circuits, including their speed, low power consumption, reliability, and low cost. Thus, the fineness of structure resolution marks one of the limits in integrated circuit technology, and a continued effort is being made to push back this limit by achieving higher degrees of resolution and by developing improved methods of forming highly resolved structures.
In the fabrication of semiconductor devices and integrated circuits, pattern definition is typically accomplished using optical, x-ray, electron beam, or ion beam lithography. The original pattern is transferred to a substrate by using a positive resist and an etching process, or a negative of the pattern may be formed on the substrate by use of a negative resist or lift-off of a deposited film. For dimensions below 1 micrometer, the mentioned processes have known limitations with respect to the formation of negative images of a pattern.
Very high resolution lithography capable of producing sub-50-nm line patterns is known to employ resist systems using deposited films of germanium-selenium with a Ag.sub.2 Se coating in combination with electron beam exposure systems, which write a pattern on the film by causing silver to diffuse into the film and form a material resistant to alkaline etching, as described in B. Singh, S. P. Beaumont, P. G. Bower, and C. D. W. Wilkinson, Appl. Phys. Lett. 41(10), 1002 (1982). Other deposited films have also been developed that are sensitive to electron beam exposure, although not all are suitable for forming micron-sized patterns, as noted, for example, in B. Singh, S. P. Beaumont, P. G. Bower, and C. D. W. Wilkinson, Appl. Phys. Lett. 41(9), 889 (1982). Electron beam sensitive films have also been mentioned in the patent art, such as in U.S. Pat. No. 4,269,934 to Borrelli et al., which discloses silver halide based films that can be written by an electron beam to form an optical mask suitable for use in microcircuit fabrication. The use of such film is in combination with a photoresist layer on a silicon wafer or other substrate, to control the exposure of the photoresist layer to visible light.
Another technique of microcircuit fabrication, somewhat similar to ion milling, is disclosed in U.S. Pat. No. 4,243,476 to Ahn et al., wherein the etching of a substrate is combined with the use of a mask of properly selected material that will release reactive gas species when struck with an ion beam. Various metal substrates can be etched by the reactive gas species, which may be released by a selected ion beam of inert gas.
It has been proposed by J. Nulman and J. P. Krusius, Appl. Phys. Lett 42(5), 442 (1983), that a satisfactory pattern inversion technique can be achieved using positive resists, local oxidation, and reactive ion etching of aluminum to transfer a pattern having dimensions below 1 micrometer. Aluminum oxide is known to have a lower ion milling rate than either pure aluminum or silicon. Aluminum films have thus been locally oxidized by use of an oxidation mask of, for example, silicon deposited over an aluminum layer in an electron beam evaporator. An initial pattern is formed in a postive high resolution resist using optical, electron beam, ion beam, or x-ray lithography. Then, the positive resist pattern is transferred to the oxidation mask on top of the aluminum film using anisotropic etching. The resulting exposed aluminum film is locally oxidized in oxygen plasma where not protected by the mask, forming an etch-resistant aluminum oxide. The aluminum film is then patterned using anisotropic etching, resulting in an inverted pattern being formed and potentially producing a useful final structure such as a gate or interconnect line. Further processing is possible, as by using the resulting aluminum pattern as a positive etch mask for patterning the underlying material.
It has been noted by J. L. Falconer, S. D. Bischke, and G. J. Hanna, Surface Science 131, 455 (1983), that an electron beam can be used with a CO.sub.2 atmosphere to enhance oxide formation on aluminum, although it is reported that the oxide is not stable unless the CO.sub.2 is removed from the gas phase while the beam continues to strike the surface.
Direct beam writing on sensitive films is known for mass storage purposes. For example, a focused laser beam may be applied to InGaSb alloy films, after which the film is explosively crystallized, causing unexposed film areas to assume a rough surface while exposed film areas maintained a smooth surface that can be read by rastering under a laser beam, all as described in C. E. Wickersham, J. Vac. Sci. Technol. A 1(4), 1857 (1983).
The examples and description of the related art as given above demonstrate the importance of very high resolution lithography in many areas of technology. Developments constituting new techniques, simplifications of existing techniques, or improvements of known techniques offer important benefits to potentially all of the mentioned areas and others, as well. To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the method of this invention may comprise the following.